Question

我们说我的查询结果如下：

ID    NAME    Phone
----  ----    -----
1     John    123456
2     John    125678
3     John    345678
4     Abby    456789
5     Abby    567890

我想只返回一个名称的单行实例：John，其中的电话号码类似于＆＃39; 12％＆＃39;。

在c＃中，我编写了这种语法来获取PersonName变量作为查询的结果。

MySqlConnection connection = new MySqlConnection("SERVER=" + "localhost" + ";" + "DATABASE=" + "testdb" + ";" + "UID=" + "root" + ";" + "PASSWORD=" + "" + ";");
MySqlCommand command = new MySqlCommand();    

    connection.Open();
    string selectQuery = "SELECT NAME FROM testtable WHERE Phone LIKE '12%' ORDER BY ID LIMIT 1";
    command.Connection = connection;
    command.CommandText = selectQuery;
    string PersonName = (string)command.ExecuteScalar();
    connection.Close();

我不知道我的代码有什么问题，但PersonName返回null。我做错了什么？

Answer 1

我们必须在这里遗漏别的东西。根据您提供的内容尝试以下代码示例：

try {
MySqlConnection connection = new MySqlConnection("SERVER=localhost;DATABASE=testdb;UID=root;PASSWORD=;");
MySqlCommand command = new MySqlCommand();    

connection.Open();
string selectQuery = "SELECT NAME FROM testtable WHERE Phone LIKE '12%' ORDER BY ID LIMIT 1";
command.Connection = connection;
command.CommandText = selectQuery;
string PersonName = (string)command.ExecuteScalar();
}
catch (Exception ex) {
MessageBox.Show(ex.Message);
}
finally {
    connection.Close();
}

我有一种感觉，由于某种原因，对.Open（）的调用失败，错误被其他地方吞没。试试上面的内容，让我知道你发现了什么。

Answer 2

执行此操作：将 (string)command.ExecuteScalar(); 更改为 Convert.ToString(command.ExecuteScalar());

MySqlConnection connection = new MySqlConnection("SERVER=" + "localhost" + ";" + "DATABASE=" + "testdb" + ";" + "UID=" + "root" + ";" + "PASSWORD=" + "" + ";");

MySqlCommand 命令 = new MySqlCommand();

connection.Open();
string selectQuery = "SELECT NAME FROM testtable WHERE Phone LIKE '12%' ORDER BY ID LIMIT 1";
command.Connection = connection;
command.CommandText = selectQuery;
string PersonName = Convert.ToString(command.ExecuteScalar());
connection.Close();

mysql命令executioncalar在C＃中返回null

2 个答案: